refactor: added some type guards

[pedrolamas]

Improved code resilience by adding some type-guards through the app.

Also removed the obsolete socket state error property.

[Copilot]

Pull request overview

This PR improves runtime resilience by adding safer type guards around JSON parsing in a few hotspots (gcode parsing, string formatting, and drag/drop data transfer), and simplifies the socket store state by removing the obsolete socket.error field.

Changes:

• Added defensive JSON parsing + runtime shape validation for polygon data and string-array parsing.
• Hardened drag/drop DataTransfer parsing by validating the payload and returning null on failure; updated consumers to handle null.
• Removed the error property from SocketState and tightened websocket error response typing.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/workers/parseGcode.ts	Adds guarded JSON parsing for EXCLUDE_OBJECT_DEFINE polygon data to avoid worker crashes on malformed input.
src/util/string-formatters.ts	Ensures getStringArray only returns parsed JSON when it’s truly a string[], otherwise falls back to splitting.
src/util/file-data-transfer.ts	Adds a type guard + try/catch around DataTransfer JSON parsing and returns null on invalid payloads.
src/store/socket/types.ts	Removes the obsolete SocketState.error field.
src/store/socket/state.ts	Drops error from the socket default state.
src/store/socket/actions.ts	Adds a helper to safely extract error messages from JSON-ish strings; types onSocketError payload.
src/plugins/socketClient.ts	Tightens websocket API error typing to SocketError.
src/components/widgets/job-queue/JobQueue.vue	Handles null from drag/drop file payload parsing before using it.
src/components/widgets/gcode-preview/GcodePreviewCard.vue	Handles null from drag/drop file payload parsing before using it.
src/components/widgets/filesystem/FileSystem.vue	Handles null from drag/drop file payload parsing and adjusts drop flow accordingly.